The only published route to 4,4'-thiobis(phthalic acid) derivatives involves treatment of 4-substituted N-alkylphthalimides with sodium sulfide followed by hydrolysis and dehydration. See Evans et al, Polymer Preprints (American Chemical Society, Div. Polymer Chem.) 1984, 25, 268. Preparation of the starting materials for this route most readily requires electrophilic substitution reactions of phthalic anhydride followed by separation of 3- and 4-substituted isomers and imide formation. See Williams et al, J. Org. Chem. 1977, 42, 3414.